Radio communications systems are widely used today, including mobile phone systems and wireless metropolitan-area networks (wireless MAN). The researchers and engineers in the field of radio communications have constantly been discussing the next-generation communications technologies to pursue higher speeds and greater network bandwidths.
In such a radio communications system, one communication device may send some control signals to another communication device. These control signals convey information about, for example, which radio resources and modulation-coding scheme to use to transmit data, so that the latter communication device can receive and decode data properly with reference to this information. The control signals may also specify which radio resources and modulation-coding scheme the latter communication device is supposed to use when it transmits data to the former communication device.
The length of control signals is not always fixed. For example, some systems define a plurality of control signal formats with different signal lengths, so that an appropriate format can be selected depending on the purpose of control signals. Another example is a radio communications system in which the length of control signals varies in accordance with the bandwidth of the frequency band that it uses for data transmission. See, for example, 3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding”, 3GPP TS 36.212 V9.0.0, 2009-12, Section 5.3.3.1.
In the case of variable-length control signals, blind decoding of received signals may be performed in the receiving end. That is, the receiving communication device is unaware of the detailed channel structure and thus makes a plurality of attempts to decode a received signal with different candidates for the length that the control signal may take. This is referred to as blind decoding. When a successful decoding attempt is reached, the receiving communication device considers it as detection of a control signal. To detect such control signals, the receiving communication device monitors a specific region of radio resources (referred to as a search space) previously defined in accordance with prescribed rules. See, for example, 3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, 3GPP TS 36.213 V9.0.1, 2009-12, Section 9.1.1.
Some radio communications systems are designed to use a plurality of frequency bands in a parallel fashion to transmit data. These individual frequency bands are called “component carriers.” A method under study further enables such systems to transmit a control signal and its associated data signals by using different frequency bands. This method, sometimes referred to as the cross-carrier scheduling, provides a carrier indicator field (CIF) to permit a single frequency band to convey control signals for a plurality of frequency bands. See, for example, 3rd Generation Partnership Project, “Way Forward on PDCCH for Bandwidth Extension in LTE-A”, R1-093699, 3GPP TSG RAN WG1 Meeting58, 2009-08.
In the case where a plurality of frequency bands are used in data communication, a plurality of control signals corresponding to these frequency bands are transmitted with one or more of those frequency bands. The receiving devices try several candidates for the control signal length in extracting information from a search space in which control signals are expected to reside. The candidates may vary depending on for which frequency band the control signals are intended. While it is possible to specify different frequency bands to send different control signals, this frequency scheduling leads to an increase in the number of candidates for the control signal length. As a result, an increased burden of control signal extraction would be imposed on the receiving radio communication devices.